The Standard model can be installed in two different ways, depending on your needs and requirements to implement a fax DID application.
As far as the SilentFax server setup is concerned, the difference between installation types is very minor and can be changed on the fly.

Non-DID installation

The Standard model can be connected directly to your existing fax lines just like having multiple fax machines in one box.
With this type of installation, each line can be assigned to a different "operator" mailbox to manually route received faxes.

DID installation

Alternatively, you can install the Standard model behind your PBX to benefit from the DID service you may have. With this installation, SilentFax can deliver received faxes to intended recipients directly, without any human intervention.

A DID installation requires your PBX system to pass through the DID digits as DTMFs. This is a common feature for PBX systems and is used to integrate with voicemail systems.